A gas turbine comprises a turbine and a compressor driven by the turbine. In particular, when the gas turbine is provided for a gas-steam power plant, the compressor is of the axial flow type. Commonly, the gas turbine is subjected to varying operating conditions resulting in different aerodynamic flow conditions within the compressor. In order to adapt the compressor performance to different operating demands, it is known to provide the compressor with variable guide vanes (VGV). The variable guide vanes are to be pivoted about their longitudinal axis in order to adjust their angle of attack.
Each variable guide vane is provided with a journal at its root, wherein the journal is pivot-mounted in a through hole in the compressor casing. The journal is phenomenal accessible from outside the compressor casing and comprises a lever to be actuated for pivoting the variable guide vane. All levers are coupled by means of a unison ring arranged concentrically around the compressor casing. The rotation of the unison ring actuates each of the variable guide vane levers simultaneously to achieve a corresponding rotational setting of each variable guide vane within the compressor casing.
An axial compressor consists of multiple stages of stator and rotor vanes (rotor blades). The front stages of stator vanes have variable pitch to control the flow. Flow control is important on engine run up to avoid surge.
Such a construction with variable pitch of the stator vanes is called “Variable Guide Vanes” (VGV).
It is known that individual vanes pitch or angular offset is controlled via a linkage mechanism comprising (see also FIGS. 1, 2 and 3):
i) A vane (10, 11) is mounted on a spindle (22) to allow angular movement of the vane.
ii) A short lever (20) connects the spindle to a driving ring (40, 41, 42, 43), the so called unison ring, all vanes in a single stage connecting to the same ring. See also FIG. 1, specifically showing individual vane (10) and lever (20).
iii) Each ring is rotated via a push rod (50) from a common bell crank (61). See also FIGS. 2 and 3, wherein FIG. 2 is showing the whole basic mechanism: Ram Drives for the stages through different amounts of rotation via the bell crank (61). FIG. 3 is showing the end view of the compressor system, as previously known.
iv) The bell crank (61) is rotated via a single hydraulic ram (60) (see again FIGS. 2 and 3).
Typically the arm through lengths of the bell crank (61) are set to give the required rotation of each unison ring and thus the angle of all the vanes on a single stage. See FIG. 4 showing a bell crank with arms (62, 63, 64) to attach the push rods to the bell crank. The arm 62 is longer than arm 63, which are again longer than arm 64.
With the use of a single driving ram the angular position during ram travel is proportional stage to stage. In some cases it may not be ideal to have a proportional system.
Non proportional operation could be achieved by several methods including individual stage rams.